I've been following the events in Japan and especially around Fukushima after the earthquake and tsunami with some interest over the past nine months or so. It punches several of my interest buttons: nuclear power, technology, engineering disasters and the subequent recovery efforts and of course Japan in general. I travelled to Japan in June last year, about three months after the reactors at Fukushima exploded releasing radioactive contamination over a wide area. I spent a day in Sendai which was badly hit by the earthquake and the tsunami as well as receiving a noticeable amount of contamination from the fallout plumes. On the way there and back from Tokyo I passed through Fukushima city which received even more direct contamination.
There are a number of online resources reporting radiation levels at various places around Japan and even in the sea close to the Fukushima site. One of the more interesting sets of figures comes from the eight fixed radiation monitors positioned around the periphery of the Fukushima Daiichi reactor complex itself. You can find these logs (in English) here. The values are, as you might expect, high but trending generally downwards over the months although the curve has flattened out quite a bit as the hot fast isotopes released in the accident have died off leaving the more long-lived residues.
The day-to-day curves oscillate up and down slightly, depending on daytime and nighttime temperature I presume. However when I looked at the recent weekly graph there was a very noticeable, if temporary dip in the measured radiation levels.
I was somewhat puzzled until I dug further into the logs which record, among other things the weather conditions. It turns out Fukushima had some snowfall at the time of the notch shown in the chart above. I presume that the snow covering the ground reduced the radiation levels being measured by all the monitoring stations. Having grown up reading a lot of SF involving post-apocalyptic nuclear war and the endless killing fallout that coats the land under such circumstances I found it suprising that a thin coat of snow (it lasted less than a day) blocked a significant amount of directly measurable radiation.